b) make it adjustable (/etc/init.d/boot mounts /dev using
"mount -n -t tmpfs -o mode=0755 udev /dev" -- I'm going
to test if the size is used when I add /dev with
a different size to /etc/fstab)...

It should honor the fstab options by re-mounting the already mounted
filesystem. We do that for some other filesystems too.

In case of a) -- which path we can use instead - /dev/.tmp?
/var may be mounted on a separate disk so it can't be used too.

Better use some name private to your package, and delete the directory after it
is no longer needed when the rootfs is available. We should not put new generic
names like ".tmp" in /dev, and suggest people to share that.